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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



THE USE OF THE SPECTROSCOPE IN THE STUDY OF 

 PLANT LIFE. 



By Eev. Prof. G. Henslow, M.A., V.M.H., F.L.S., &c. 



[Read April 5, 1910.] 



Experiments to test the effects of light upon the growth of plants 

 have been frequently made from the end of the eighteenth century to 

 the present day, and, during the last sixty years, with the addition of 

 coloured media, to ascertain the relative effects of the different rays of 

 the solar spectrum. The results have' never been more than approxi- 

 mate only, as on the one hand so many different processes are going 

 on simultaneously in the plant which interfere with any attempt to find 

 out the actual conditions of any one of them; and secondly, the rays 

 cannot be sufficiently isolated so as to use a pure monochromatic light. 

 The results of experiments have therefore not altogether unexpectedly 

 been contrary. 



I propose, therefore, only to give the general conclusions arrived at 

 and some results of my own experiments. 



The three chief functions of plant life excited into action by the 

 sun's rays are Respiration, or the process similar to our own breathing, 

 involving the absorption of oxygen and the giving off of carbonic acid 

 gas ; Transpiration, or Exhalation, as it is sometimes called, the giving 

 off of the vapour of water; and Assimilation, or the absorption and 

 decomposition of carbonic acid, whereby the carbon is retained, the 

 first visible product being starch, while the oxygen is given back to the 

 air. All these functions are performed by living protoplasm ; but with 

 this difference, that while respiration is stimulated by heat-rays, and 

 may go on independently of the luminous rays, as in seeds germinating 

 in darkness ; and transpiration does not cease, though it is lessened 

 when plants are blanched in total darkness, as sea-kale; yet this func- 

 tion as well as assimilation is, in normally green plants, dependent upon 

 the direct action of the solar rays, which are absorbed by the green 

 colouring matter of the chlorophyll grains. 



To render visible their absorption of light, an alcoholic or other 

 solution of chlorophyll must be observed through a three-sided prism 

 which decomposes white light; when the spectrum " is thus examined 

 with a sufficiently concentrated solution seven dark bands may be seen 

 crossing it. This means that those rays of light cannot penetrate the 

 green colouring matter, but are absorbed by it. 



In a paper read before this Society on March 14, 1893, and printed 

 in the Journal (vol. xvi., p. 89), I dealt wath the effects of growung 

 plants under glasses of different colours. The conclusion arrived at 

 was that the yellow, blue, and colourless or clear glass were most 



